1. Field of the Invention
The invention relates to a control apparatus and control method of a vehicular automatic transmission. More particularly, the invention relates to control of an automatic transmission when tie-up occurs in the automatic transmission.
2. Description of the Related Art
Vehicles provided with an automatic transmission in which a shift is executed by selectively engaging a plurality of engagement devices are well known. For example, the vehicle is described in Japanese Patent Application Publication No. JP-A-2000-65203 is one such vehicle. Japanese Patent Application Publication No. JP-A-2000-65203 discloses a control apparatus of an automatic transmission which prevents tie-up (i.e., double lock-up) from occurring in an automatic transmission. More specifically, if an engagement device is unable to release due to a problem with, for example, a shift control valve that controls the engagement operation of the engagement device, it may cause tie-up to occur in the automatic transmission 10. When this kind of tie-up cause is detected, i.e., when abnormal engagement of an engagement device is detected, the control apparatus prevents that tie-up from occurring by unconditionally establishing a specific speed set in advance that includes, as a necessary condition in order for that specific speed to be established, the engagement of the engagement device that would cause the tie-up to occur.
The shift into the specific speed to be established which includes, as a necessary condition in order for it to be established, the engagement of the engagement device may be a downshift. Typically when a downshift is executed in an automatic transmission when a vehicle is running, the engine speed increases in response to the change in speed ratio of the automatic transmission before and after that downshift in which there is almost no change in the vehicle speed.
As a result, depending on the running state of the vehicle, i.e., depending on the vehicle speed, for example, there is a possibility that the engine speed may exceed the maximum speed allowed for the engine if a downshift is performed into the specific speed to be established that includes, as a necessary condition for that specific speed to be established, the engagement of the engagement device that will cause tie-up to occur.